Im starting to feel like a broken record here, , but I've got yet another question about my 1986 F-150 5.0l 302cid EFI.

So I've finally solved the problems in my other three posts (well mostly) and now I'm at the last problem preventing the truck from firing up. No injector pulse. (tested with multiple noid lights)

The only things I can think of that would prevent the injector pulse is a bad EEC, a bad ground, or a broken wire in the injector harness. The truck will start up consistently with starting fluid.

Now, I don't think it is the EEC because I get power to everything else on the truck and the fuel pumps run as they should (been told that the injectors run off the same), I have pulled codes and I get two codes about the TPS and EGR and thats it.
I dont think it is a broken wire because I've taken all the tape off the wires back to the firewall/to computer and found no breaks or frays.
That only leaves a ground problem, but I cannot for the life of me find the bad ground. I have a haynes manual and have looked at the wiring diagrams in the back to no avail.

Where is the exact location of the ground wire that the computer uses to switch the injectors on and off? I have also replaced the ignition module. I do get 12v to both sides of injector connector.

 

The computer uses it's own main ground. If I remember right it comes from over near the battery.

The EEC relay sends constant voltage out to all the injectors. There are individual wires going back to the computer from each injector, and the computer grounds this wire to make the injector spray.

 

Edit; The trucks are batch fired, all the injectors fire half the fuel the engine needs, and then on the next revolution, the other half is injected. According to the diagram below, there are two main circuits from the computer to the injectors, 4 injectors per circuit.

 

I just thought of something; The computer has a "clear flooded engine" mode. If the engine is turning below a certain rpm, and the TPS tells it that you are holding the pedal to the floor, it turns the injectors off. Maybe you had better fix that TPS code first. I wonder if the TPS is telling the computer you are holdng the pedal to the floor when you really are not.

 

Ah! I had wondered the same thing, but I wasn't sure that could happen. I will definitely look into that.
Also, thanks for the clarification on how the injectors fired, I knew the computer wasn't able to ground out each individual injector based on that wiring diagram, but thats what I was hearing. Glad to finally know for sure how it works.

My only options as I see it now are that TPS sensor and somehow a tiny short occurred only in the computer preventing the injectors from pulsing. I figure this because everything else the computer controls is working correctly and there are no breaks or frays in the harness. I will let you know how it goes with the TPS in a few days.

 

The mustang engines do have 8 wires to the computer. They are called "SFI" or Sequential Fuel Injection.

 

Caleb - I have seen two of these OBD-I vehicles not fire the injectors when the crankshaft position sensor was malfunctioning.... not trying to say that is your problem, just something you should be aware of....

 

Thanks for the comment! All ideas are welcome and its nice to not have to look around forever for an answer if the tps or computer end up not being the problem.

Dave, thanks for the info. It always helps to know more. I will post on here again, hopefully this weekend, with the results.

 

These old trucks don't have a crankshaft position sensor, they use the pickup inside the dist for engine position info. I think one of the teeth on the wheel inside is a little wider than the others.

 

If you don't really know what shape your distributor is in, you should take it out and have a good look at it. Especially where the TFI module terminals attach to the pickup coil. If it looks bad it could be your problem like ctubutis said.



But the easiest thing to do would be to replace the EEC relay which is somewhere behind your ash tray inside the cab.

 

I will take a look at my distributor as well as the tps. I replaced the TFI module last weekend without looking at the connectors, but everything felt snug and the old pins didn't have a trace of corrosion on them. I assumed it was working correctly because I can consistently start the engine on ether & jumper cables ( for a second or two) and I have good spark.
I will post again when I've checked out everything suggested here. Unfortunately it might be a week or two as I am back at school and have to pay rent soon.

 

Fixed the TPS problem, ended up being a loose wire. Rechecked codes today after firing it over a few times on ether and got a KOEO code 14, haynes says it's a PIP fault. Just replaced the ignition module last weekend so it's most likely not that. Assuming it's the sensors inside the distributor.

Ctubutis mentioned that he had seen two vehicles not fire the injectors when the crankshaft position sensor failed (pickups inside the dist). Sounds like I will be buying a distributor.

 

Noticed something new!

I was just messing with the truck and decided to give the accelerator short, quick taps to see if it would do anything... and lo and behold it did.

I would first start the engine with ether and then press the accelerator as described above. The engine would run alright on the ether, and then as the ether burned off, the engine would start to sputter and pop (kinda like a backfire??) for around 5-7 seconds more than if I had not depressed the accelerator. The smell changed as well.

Could this be fuel getting to the engine? If so, why in the hell would this happen, and does the popping indicate bad timing?

I should note: The engine turns over pretty slowly and the positive and starter cable get VERY hot. I'm assuming this is the starter going bad, but if you have another idea please tell.

 

The computer uses the signals from the distributor to fire the injectors. It's the only way it gets rpm information. It also uses these signals to verify the engine is actually turning, and to activate the fuel pump.

If you had a loose wire at the tps, I would also check for a loose wire going to the distributor. Some of these important dist wires are shielded, Make sure one of the wires is not bare and shorting against the shield.

I think if you get rid of the code 14, it will run.

 

Alldata DIY says code 14 is loose connections, bad wire, or arcing ignition components.